In mass spectrometers, an ion optical system, which is also referred to as an ion lens or ion guide, is used to converge ions supplied from the previous stage, accelerate them in some cases, and send them to a mass analyzer (e.g. a quadrupole mass filter) in the next stage. Some conventional ion optical systems use multipole rod configurations, such as a quadrupole or octapole system. In the case of a quadrupole mass filter, which is often used as a mass analyzer for separating ions according to their mass, a set of short pre-rod electrodes may be additionally provided before the main quadrupole rod electrodes to smoothly introduce ions into the main rod electrodes. Furthermore, a set of short post-rod electrodes may also be provided after the main quadrupole rod electrodes to prevent the movement of ions from being disturbed due to a disturbance of the electric field at the rear end of the quadrupole rod electrodes. These pre-rod and post-rod electrodes are also a type of ion optical systems.
FIG. 15(A) is a schematic perspective view of a common type of quadrupole rod ion guide 710, and FIG. 15 (B) is a plan view of this ion guide 710 in an x-y plane perpendicular to its ion beam axis C. The ion guide 710 includes four cylindrical rod electrodes 711-714 arranged parallel to each other so as to surround the ion beam axis C. Normally, as shown in FIG. 15(B), the pair of rod electrodes 711 and 713 opposing each other across the ion beam axis C are supplied with a radio-frequency voltage V·cos ωt, while the other pair of rod electrodes 712 and 714 neighboring the first pair around the ion beam axis C are supplied with a radio-frequency voltage V·cos(ωt+π)=−V·cos ωt, i.e. a voltage having the same amplitude as that of the first radio-frequency voltage V·cos ωt with a phase shift of 180 degrees (i.e. with a reversed polarity). Applying the radio-frequency voltages ±V·cos ωt in this manner creates a quadrupole radio-frequency electric field within a space surrounded by the four rod electrodes 711-714. Within this electric field, ions are transported to the next stage while being oscillated and converged close to the ion beam axis C.
FIG. 16 is a plan view of an octapole rod ion guide 720 in an x-y plane perpendicular to its ion beam axis C. The eight cylindrical rod electrodes 721-728 are arranged at equal angular intervals around the ion beam axis C and in contact with an inscribed cylinder A. The manner of applying radio-frequency voltages to these rod electrodes 721-728 is the same as in the case of the quadrupole configuration: the same radio-frequency voltage is applied to any two rod electrodes opposing each other across the ion beam axis C, and two radio-frequency voltages having a phase shift of 180 degrees are respectively applied to any two rod electrodes neighboring each other around the ion beam axis C.
In the multipole rod ion optical system with four or more poles, the profile of the radio-frequency electric field formed within the space surrounded by the rod electrodes changes depending on the number of poles. This change is accompanied by some changes in the ion optical characteristics, such as the ion beam convergence, ion transmission efficiency, ion acceptance, ion-storing capacity and mass-separating capability. In general, using a smaller number of poles improves the beam convergence and mass-selecting capability due to the cooling effect caused by a collision with neutral molecules; increasing the number of poles lowers the beam convergence and mass-selecting capability while improving the transmission efficiency and acceptance of the ions.
Patent Documents 1, 2 and other documents disclose an ion optical system using virtual rod electrodes. FIG. 17 is a schematic configuration diagram of an ion optical system using virtual rod electrodes. In this ion optical system 730, the rod electrodes 711, 712, 713 and 714 shown in FIG. 15(A) are respectively replaced with four virtual rod electrodes 731, 732, 733 and 734, each of which is composed of a plurality of plate electrodes 735 arranged along the ion beam axis C. (Although four plate electrodes are used in the example of FIG. 17, this number can be arbitrarily changed.) The radio-frequency voltages applied to these virtual rod electrodes 731-734 are the same as those applied to the real rod electrodes 711-714 shown in FIG. 15(B).
However, in the case of the virtual rod electrodes 731-734, it is possible to apply a different voltage to each of the plate electrodes forming one virtual rod electrode. Therefore, for example, a DC voltage that increases in a stepwise manner along the moving direction of the ions is superimposed on the radio-frequency voltage. The DC electric field created by this DC voltage has the effect of accelerating or decelerating the ions passing through the space surrounded by the virtual rod electrodes 731-734. Thus, the acceleration or deceleration of the ions can be easily performed. Furthermore, in the present configuration, the plate electrodes forming one virtual rod electrode can be arranged so that they come closer to the ion beam axis C with the movement of the ions. In this configuration, the space within which the ions can oscillate becomes smaller with the movement of the ions. Consequently, the ions are converged closer to the ion beam axis C so that, for example, they can be efficiently guided through a small hole formed at the tip of a skimmer and transported to the next stage.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-149865    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-351563